Computers commonly use disc drives for memory storage purposes. Disc drives include a stack of one or more magnetic discs that rotate and are accessed using a head or read-write transducer. Miniature hard disc drives are smaller than other hard disc drives and are used in hand-held electronic devices and portable electronic devices such as cellular phones, hand-held personal computers, digital cameras and personal digital assistants (PDA's). Typically, a high-speed motor such as a spindle motor is used to rotate the discs. An example of a miniature hard disc drive is International Business Machines' (IBM) Microdrive™.
An example of a conventional spindle motor 1 used in a hard disc drive is shown in FIG. 1. The motor 1 includes a base 2 which is usually made from die cast aluminum, a stator 4, a shaft 6, bearings 7 and a disc support member 8, also referred to as a hub. A magnet 3 and flux return ring 5 are attached to the disc support member 8. The stator 4 is separated from the base 2 using an insulator (not shown) and attached to the base 2 using a glue. Distinct structures are formed in the base 2 and the disc support member 8 to accommodate the bearings 7. One end of the shaft 6 is inserted into the bearing 7 positioned in the base 2 and the other end of the shaft 6 is placed in the bearing 7 located in the hub 8. A separate electrical connector 9 may also be inserted into the base 2.
Each of these parts must be fixed at predefined tolerances with respect to one another. Accuracy in these tolerances can significantly enhance motor performance.
In operation, the disc stack is placed upon the hub. The stator windings are selectively energized and interact with the permanent magnet to cause a defined rotation of the hub. As hub 8 rotates, the head (not shown) engages in reading or writing activities based upon instructions from the CPU in the computer.
Manufacturers of disc drives are constantly seeking to improve the speed with which data can be accessed. To an extent, this speed depends upon the speed of the spindle motor, as existing magneto-resistive head technology is capable of accessing data at a rate greater than the speed offered by the highest speed spindle motor currently in production. The speed of the spindle motor is dependent upon the dimensional consistency or tolerances between the various components of the motor. Greater dimensional consistency between components leads to a smaller gap between the stator 4 and the magnet 3, producing more force, which provides more torque and enables faster acceleration and higher rotational speeds. One drawback of conventional spindle motors is that a number of separate parts are required to fix motor components to one another. This can lead to stack up tolerances which reduce the overall dimensional consistency between the components. Stack up tolerances refers to the sum of the variation of all the tolerances of all the parts, as well as the overall tolerance that relates to the alignment of the parts relative to one another.
An important characteristic of a hard drive is the amount of information that can be stored on a disc. One method to store more information on a disc is to place data tracks more closely together. Presently this spacing between portions of information is limited due to vibrations occurring during the operation of the motor. These vibrations can be caused when the stator windings are energized, which results in vibrations of a particular frequency. These vibrations also occur from harmonic oscillations in the hub and discs during rotation, caused primarily by non-uniform size media discs.
An important factor in motor design is the lowering of the operating temperature of the motor. Increased motor temperature affects the electrical efficiency of the motor and bearing life. As temperature increases, resistive loses in wire increase, thereby reducing total motor power. Furthermore, the Arhennius equation predicts that the failure rate of an electrical device is exponentially related to its operating temperature. The frictional heat generated by bearings increases with speed. Also, as bearings get hot they expand, and the bearing cages get stressed and may deflect, causing non-uniform rotation and the resultant further heat increase, non-uniform rotation requiring greater spacing in data tracks, and reduced bearing life. One drawback with existing motor designs is their limited effective dissipation of the heat, and difficulty in incorporating heat sinks to aid in heat dissipation. In addition, in current motors the operating temperatures generally increase as the size of the motor is decreased.
Manufacturers have established strict requirements on the outgassing of materials that are used inside a hard disc drive. These requirements are intended to reduce the emission of materials onto the magnetic media or heads during the operation of the drive. Of primary concern are glues used to attach components together, varnish used to insulate wire, and epoxy used to protect steel laminations from oxidation.
In addition to such outgassed materials, airborne particulate in a drive may lead to head damage. Also, airborne particulates in the disc drive could interfere with signal transfer between the read/write head and the media. To reduce the effects of potential airborne particulate, hard drives are manufactured to exacting clean room standards and air filters are installed inside of the drive to reduce the contamination levels during operation.
With the rapidly expanding development of personal computers into the field of first what was termed portable, then lap-top, notebook and now hand held size computers and digital cameras, there has been a tremendous demand for smaller disc drives with increased performance for such small computers. Especially important to manufacturers, is the ability to reduce the height of the disc drive so that the size of the casing for the computer could be minimized. It is an objective of the present invention to provide a compact hard disc drive system that is compatible with notebook and hand held computer applications, and can be compatible with devices using Type I and Type II Flash memory devices.
Another objective of the invention is to provide a compact hard disc drive system that has lower vibration and greater structural integrity to provide increased data storage capability and increased speed.
Another example of a spindle motor that can be used in a hard drive is shown in U.S. Pat. No. 5,694,268 (Dunfield et al.) (incorporated herein by reference). Referring to FIGS. 7 and 8 of this patent, a stator 200 of the spindle motor is encapsulated with an overmold 209. The overmolded stator contains openings through which mounting pins 242 may be inserted for attaching the stator 200 to a base. U.S. Pat. No. 5,672,972 (Viskochil) (incorporated herein by reference) also discloses a spindle motor having an overmolded stator. One drawback with the overmold used in these patents is that it has a different coefficient of linear thermal expansion (“CLTE”) than the corresponding metal parts to which it is attached. Another drawback with the overmold is that it is not very effective at dissipating heat. Further, the overmolds shown in these patents are not effective in dampening some vibrations generated by energizing the stator windings.
U.S. Pat. No. 5,806,169 (Trago) (incorporated herein by reference) discloses a method of fabricating an injection molded motor assembly. However, the motor disclosed in Trago is a step motor, not a high-speed spindle motor, and would not be used in applications such as hard disc drives. Furthermore, none of these prior art embodiments integrate the base of the hard disc drive, thereby eliminating the cost of the base. Thus, a need exists for an improved miniature hard disc drive that is small and lightweight yet overcomes the aforementioned problems.